Many chemical reactions involve the release of significant amounts of heat, which is preferably removed at least partially to avoid impairment of the progress of the desired reaction itself or for other reasons such as product quality or equipment integrity. In the design of many chemical reactors a principal problem is temperature control. This applies in particular to oligomerization reactors, which are often also referred to as polymerization reactors. Some of the large exothermic heat of reaction may be absorbed by heating up the reactants, depending upon the reaction system, but other means are often used to dissipate the rest. To achieve this end, two types of reactors are typically employed with solid catalysts: chamber and tubular, such as generally described in Hengstebeck, “Petroleum Processing”, McGraw-Hill (1959), pp. 208-234.
Chamber reactors are typically vertical cylindrical vessels containing several beds of catalysts, with provisions for injecting a cooler quench liquid between them. The use of a chamber reactor for the oligomerization of light olefins to heavier olefins is described, for instance, in U.S. Pat. No. 6,072,093.
Tubular reactors typically are single-pass heat exchangers (e.g., shell-and-tube), with the catalyst normally contained in the tubes. The shell side typically contains a circulating heat exchange fluid. For reasons of more effective heat transfer, it is often preferred to select this fluid such that shell-side conditions can be applied under which the selected fluid at least partly evaporates. A convenient selection is in many instances to use water/steam because inter alia water is readily available, the temperature of the reactor can be controlled by controlling steam pressure, and the system is readily integrated with the water/steam systems typically present in many chemical and petrochemical operations. The use of tubular reactors for the oligomerization of light olefins to heavier olefins is described, for instance, in U.S. Pat. No. 4,709,111.
For any given operation, tubular reactors are typically more expensive to build, take longer to charge and discharge catalyst, and operate with higher concentration of reactants, compared with chamber reactors. Accordingly, it is required that temperature be more closely controlled. This is an area of active research in the industry. See, for instance, U.S. Patent Application Nos. 20030133858, 20040266893, and 20050061490.
Additional references of interest include U.S. Pat. Nos. 4,141,937, 4,456,781, 6,072,093, 6,846,966, WO 200500534, WO 2005026640, and Jafari Nasr and Tahmasebi, “Application of Heat Transfer Enhancement on Vertical Thermosyphon Reboilers Using Tube Inserts”, presented at the 16th International Congress of Chemical and Process Engineering, 22-26 Aug. 2004, Prague, Czech Republic
Reactor design in a chemical process often is directed to approaching isothermal operation of exothermic reactions. At optimal conditions, vigorous boiling of the liquid, e.g. water, in the shell occurs across the entire surface of the tubes. In a vertical tubular reactor where coolant (e.g. water) enters at the bottom and exits the top, in an inefficient operation the tubes are immersed in a non-boiling liquid phase at the bottom and are contacting primarily or even only vapor (e.g. steam) at the top. Heat transfer efficiency is therefore impaired at top and bottom, and can drop by roughly a factor of 10 relative to optimal conditions. It is extremely difficult to achieve optimal heat transfer conditions with current design practice.
Current reactor design is shown in FIG. 1. The heat exchange fluid is assumed to be water. Water/steam exiting the reactor (not shown) enters via conduit 1 into (or near) the top of a conventional steam drum 10. The water in conduit 1 either drops into the steam drum 10 or is entrained in the steam produced by the heat of reaction and removed along with steam via conduit 2. The steam and entrained water that exits the steam drum 10 via conduit 2 is replaced by make-up water or boiler feed water (BFW) which is fed into the steam drum 10 via conduit 3, typically entering below the water level illustrated by line 11 in FIG. 1. The water entering conduit 3 typically is deaerated by bringing it close to or up to boiling temperature at about atmospheric pressure, and therefore substantially colder than the water already in the drum, where the pressure is typically higher. Water exiting steam drum 10 via conduit 4 is returned to the reactor. As would be recognized by one of ordinary skill in the art, passage through steam drum 10 is induced by at least one of (i) forced flow, such as by mechanically pumping said water/steam, preferably the water in conduit 4, and (ii) thermosyphon circulation.
The present inventors recognized that with this design the temperature difference between the water exiting steam drum 10 and entering the reactor via conduit 4 is substantially different than the temperature of the water exiting the reactor and entering steam drum 10 via conduit 1. Accordingly, isothermal operation is difficult if not impossible to achieve. In such a situation the fluid entering the reactor is subcooled and will not start boiling immediately upon heating. This results in inefficient heat transfer. Approaching isothermal conditions is more readily achieved if the temperature of the water entering the reactor is closer to the temperature of the water exiting the reactor.
One solution has been to preheat the make-up water entering via conduit 3 using an external source of heat. However, such a solution has two problems. First, it is almost always an expensive solution to preheat water in this manner. Second, when only one preheat system and multiple steam drums are in use, it was (heretofore) difficult or impossible to achieve isothermal reactor temperatures in all the reactors simultaneously.
The present inventors have surprisingly discovered an efficient method of maintaining isothermal operation of a reactor by having the water/steam exiting said reactor and the makeup water enter the same phase of the steam drum, i.e., both enter into the liquid phase or both enter into the vapor space above the liquid phase. The inventors have also discovered a method of maintaining isothermal operation of a system comprising multiple reactors.